This application claims the priority of German patent document 196 50 477.5, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for measuring the braking moment in a motor vehicle.
A measuring device of this generic type, disclosed in British Patent Document GB 2 039 063 A1, incorporates a two-part measuring hub, with both parts being designed as annular discs. The first part consists of an inner section located coaxially to the wheel axis and near the axle, and releasably connected with a motor vehicle wheel rim by a screw connection. The outer section, located concentrically with respect to the inner section and spaced radially therefrom, forms the marginal area of the measuring hub and is integrally connected therewith by ribs in the form of spokes extending radially. Measuring strips that detect expansion stresses are fastened to the ribs, with the forces acting on the wheel in the circumferential direction being measured by the strips on the basis of the shear stress developed in the ribs. The outer annular section also has protrusions extending radially inward between the ribs, the ends of which protrusions each form a gap with the inner section, extending in the circumferential direction.
The second part has four projections which extend perpendicularly edgewise from the disc plane, and are offset from one another in the circumferential direction by an angle of approximately 90.degree.. With the measuring hub in the assembled state, the projections pass axially from the inside to the outside through the gaps and are welded to the protrusions. The projections are designed so that they delimit a very narrow gap with the inner section of the first part in the operating position of the second part, so that when a given maximum twisting of the measuring hub spokes in the circumferential direction, (determined by the width of the gap) is exceeded during measurement by the force moments acting at this point, overload protection is provided by the arrangement of the spokes at the protrusions and/or the projections of the second part. The second part is bolted to an adapter, which in turn then is bolted in the direction of the motor vehicle body adjacent to a disk of a disk brake connected to the wheel hub. The brake is mounted on the wheel axle coaxially to the measuring hub.
This measuring hub is used to measure torques and is mounted between the rim and the hub so that it detects all possible torques acting on the wheel, and a total torque is recorded, including for example, frictional moments from the axle bearing and the transmission, braking moments from the disk brake, as well as the wheel load and the lateral forces on the wheel. However it is impossible to detect, in a sensitive and selective fashion, only the resultant forces that act on the brake disk as a braking moment. It is also not possible to design these measuring hubs for very small braking moments since the measuring hubs must have sufficient strength, especially at the measurement points, to be able to withstand the wheel loads and lateral forces when driving. Conventional measuring hubs are therefore completely insensitive to small braking moments.
A measuring hub disclosed in German patent document DE-OS 2 302 540 has ribs provided with expansion-measuring strips and fastened endwise to a brake drum at one end and to a vehicle wheel at the other by screw connections, with no individual adaptation of the measuring hub to the object to be measured being required apart from the geometric arrangement of the through holes for the connecting screws.
A system for torque measurement in motor vehicles is disclosed in German patent document DE 93 01 111.3 U1, in which a brake disk is mounted on a vehicle hub. A measuring hub with a flange projecting radially inward is screwed to the disk by means of an adapter. Another adapter is screwed to the measuring hub in its circumferential area, with a test wheel being fastened to the adapter by screw connections. This makes it possible to obtain test results on vehicles that can be transferred to similar vehicles with wheels mounted directly on the vehicle hubs.
In addition, in a torque-measuring system disclosed in German patent document DE 27 08 484 C2, influencing of the torque-measuring result by significantly fluctuating axial or radial loads on the torque-measuring device is prevented. For this purpose a torque-measuring disk is fastened by screw connections to a wheel flange that can be braked and is connected with the wheel hub. The flange has a section which is located radially inward and is separated by a hole circle from the outward area of the measuring disk. The holes, located close together on a circular line, are separated from one another by narrow ribs, to which ribs expansion measurement strips are glued laterally, in other words in the walls of the holes, at the points of maximum bending stress. The ribs connect the inner section with the outer section of the measuring disk. The outer section in turn is mounted to a wheel hub by screw connections. Since tensile or compressive stresses induced by horizontal and vertical radial forces make themselves felt at points on the ribs other than those to which the expansion-measuring strips have been applied, these measurement-distorting forces are screened out, so to speak, during measurement. At the same time, axial forces act on the rib formation only to a negligible degree. Thus, deformations of the measurement ribs that are caused exclusively by torque are recorded.
In addition, a measuring hub for measuring forces and/or moments acting axially and/or radially on a vehicle wheel disclosed in German patent document DE-AS 2 104 003. The hub is flanged in place of the wheel bearing to the vehicle axle, fixed in the rotational direction. By means of bearings located inside the measuring hub, the hub rotatably supports the vehicle wheel. The braking and traction moments thus cannot be determined selectively. The measuring hub has three different circles of measuring ribs in the form of a spoked wheel. These ribs are located axially at different positions relative to the central plane of the wheel in part differently with respect to the rotational axis. Expansion-measuring strips are mounted on the spokes for measuring the tensile and/or compressive forces in the area of the smallest bending moments.
A device for measuring forces and moments acting on vehicle tires can be found in German patent document DE 37 15 472 C2, in which a measuring disk is mounted endwise by screws to an inwardly located section (hub part) on an axle flange of a wheel axle. An outer section (rim section) containing the marginal area of the measuring disk is connected with the hub part by narrow ribs to which expansion-measuring strips are glued. The ribs are formed by U-shaped slots machined into the disk in a circular arrangement at 90.degree. intervals from one another and by U-shaped slots in the disk that are offset by an angle from the first slots and are spaced 90.degree. apart from one another, the latter slots being in the shape of an inverted U. The rim part is bolted to a wheel rim.
A torque-measuring disk disclosed in U.S. Pat. No. 3,298,223 is mounted to a wheel rim by an outer section, and to a wheel hub by an inner section. In addition, a brake disk of a braking device of a motor vehicle is fastened by bolts separately to the wheel hub. A sensor for detecting the acting tensile and/or compressive forces is fastened between the outer section of the measuring disk and a torque lever flanged directly to the wheel hub.
Finally, a torque sensor disclosed in European patent document EP 0 575 634 A1, consists of two coaxial circular ring flanges which are arranged sequentially and parallel to one another and connected together by a plurality of axial ribs. Expansion-measuring strips are mounted on the ribs, by which strips the bending expansion of the rib caused by the applied torque is determined. In another embodiment, the sensor is designed as a spoked wheel, with the expansion-measuring strips mounted on the insides of the spokes. In a third embodiment sections in the form of circle segments project radially from the wheel hub into the spaces between the spokes. These sections provide overload protection when the expansion load limit of the spokes is reached.
The goal of the present invention is to provide an improved measuring device of the type described above, in which braking moments acting on the disk brake can be determined separately from other moments acting on the motor vehicle wheel, in a simple fashion and without any overlap.
This and other objects and advantages are achieved by the measuring apparatus according to the invention, which incorporates a measuring hub mounted on a wheel axle and connected with a wheel rim. Viewed in the radial direction, the hub is divided into an inner part close to the axle and an outer part that forms the marginal area of the measuring hub and surrounds the inner part concentrically. These two sections are connected to one another by ribs which extend radially, with sensors mounted thereon for measuring the bending stress acting on the ribs as a result of the introduction of a torque. In order to determine the braking moments acting on the disk brake without superimposition of other moments acting on the vehicle wheel, the brake disk of a braking device of the motor vehicle surrounds the outer part of the hub concentrically, and is fastened only to the outer part of the hub.
Thanks to the invention, only the braking moments that result from the braking device (those resulting from the interaction of the brake caliper and the brake disk) act on the ribs connecting the two sections of the measuring hub. Because of the arrangement of the brake disk and of the outer section, being separated by the ribs and completely decoupled from the inner part (and hence from the wheel axle), these braking moments can be detected simply by the sensors applied to the ribs, without superimposition of other torques caused by bearings and by the transmission which otherwise would distort the measurement results. Such torques are essentially screened out by the sole connection of the brake disk to the outer section of the hub.
Because the braking torques are detected without distortion, both small braking moments as well as large braking moments that result for example from a full brake application can be detected equally well. Moreover, by virtue of the optimally short power flow path isolated from other influences, it is especially suitable for measuring very small braking moments with great sensitivity, for example the frictional moments that result from the action of the brake caliper on the brake disc when the brake is not being applied. This residual friction of the brakes, which results from a non-uniform thickness distribution and/or a stick-slip effect of the brake disk on the brake caliper, causes constant and considerable power losses and should therefore be as low as possible. By measuring the residual friction (in other words, by detecting the small braking moment), a rapid and reliable evaluation can be made as to whether design changes in the braking device will achieve the desired result, namely reducing residual frictional moments.
In the measuring hub according to the invention, due to the arrangement of the brake disk on the measuring hub, the only torque which affects the ribs (containing the measurement points between the part) are the braking torques that result from the interaction of the brake disk with the brake caliper. Therefore, the ribs can be designed with a strength that needs only withstand the braking torques. Because of the lower demands on the strength of the measurement points, the ribs can be designed with a smaller thickness and/or greater length than conventionally, so that they are very flexible by comparison, considerably increasing measurement sensitivity.
On the other hand, measurement with conventional measuring hubs is possible only with considerable inaccuracy, since the measuring hubs must withstand all of the forces and moments acting on the wheel. This leads to compromise solutions in designing measuring hubs because of the high strength requirements, being made very sturdy at the measurement points as well, which is disadvantageous for measurement sensitivity, especially when measuring small moments.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.